This invention relates to a method for converting feedlot animal waste material into useful products including methane and an animal feed material. In another aspect, this invention relates to a method for subjecting feedlot waste materials to a two-stage fermentation process to produce a methane containing gas and a material that can be utilized as an animal feed. In still another aspect, this invention relates to a process wherein feedlot waste materials are subjected to an anaerobic fermentation process to produce methane and a first residue material followed by an aerobic fermentation of the first residue material to produce a protein rich material that can be used as an animal feed. In yet another aspect, this invention relates to a process wherein feedlot waste materials are subjected to an anaerobic fermentation process in the presence of added nitrogen to produce a residue material having a carbon to nitrogen ratio that is significantly lower than the feedlot waste material.
It has been estimated that there are on the order of 115 to 120 million head of beef cattle in the United States. These cattle produce vast quantities of fecal waste material.
In recent years, a new industry has developed in which beef cattle are placed in feedlots, which are rather confined areas, and the cattle are fed specialized rations to increase the body weight of the animals and improve the quality of the carcass of the animals. It has been estimated that from 10 to 20% of all beef cattle in the United States are maintained in feedlots and that the cattle so maintained in feedlots produce from 130 million to 260 million tons of dry organic fecal waste materials per year.
With the production of vast quantities of fecal waste materials by feedlot operations, it will be apparent that the economic disposal of such waste materials is a growing problem. Until very recently, the most common way of disposal of such waste materials was to simply let the waste material decompose by natural processes in the feedlot. The inaction of feedlot owners in processing fecal waste materials from feedlots has led to some rather serious problems in the area of pollution of air and water resources due to the stench of the decomposing fecal waste materials and the runoff and percolation of contaminated water from the feedlots. Additionally, serious health problems have been observed due to the attraction of insects and other pests that feed upon and multiply in the fecal waste materials.
Rather crude and elementary methods of minimizing or eliminating the problem of feedlot fecal waste material buildup have been suggested. Among these solutions is the rather obvious solution of merely hauling the fecal waste materials away and dumping it on fields to add nutrients to the soil. Additionally, composting of the fecal waste material has been undertaken on a limited scale to decompose the fecal waste material and sell it as an agricultural fertilizer component. These rather crude methods of disposing of fecal waste materials from feedlots have not met with great commercial success because of increasing transportation costs, complicated handling procedures involved in hauling and handling the materials, and almost nonexistent returns on capital investment in such processes.
Recently, two important factors have developed in the feedlot industry that affect the past methods for disposal or nondisposal of feedlot waste materials. The first factor is the increasing pressure from environmental groups and the public to develop safe and efficient methods for disposing of the feedlot waste materials to prevent air and water pollution and to minimize the health dangers posed by the accumulation of large amounts of fecal waste materials. The second important development is in the area of economics. The margins for profits in the area of feedlot operations continue to decrease because of the increased cost of feed materials and the failure of meat prices to increase at the same rate the feed prices are increasing. Thus, methods for decreasing purchased feed usage and methods for converting waste materials into useable valuable materials are very attractive to feedlot operations.
Several types of processes for converting cattle manure into more valuable products have been recently suggested. One such method has been disclosed in U.S. Pat. No. 3,838,199 wherein a process for thermophilic anaerobic fermentation of cattle manure is said to produce a feed supplement. This process has not been widely used because of the very high energy consumption involved in evaporating the broth and because the amount of material recovered from the broth for use as a feed supplement is very small. Additionally, the method disclosed in the patent is not particularly applicable for the conversion of feedlot fecal waste, especially in a continuous process.